In internal-combustion engines, camshafts are used for activating the gas-exchange valves. The camshaft is mounted in the internal-combustion engine such that cams mounted on the shaft contact cam followers, for example, cup tappets, rockers, or valve lifters. If the camshaft is set in rotation, then the cams roll on the cam followers, which in turn activate the gas-exchange valves. Thus, the position and the shape of the cams set not only the opening period and also the amplitude, but also the opening and closing times of the gas-exchange valves.
Modern engine concepts tend toward designing a variable valve drive. On one hand, valve stroke and valve opening period should be able to be formed variably up to complete deactivation of individual cylinders. For this purpose, concepts such as switchable cam followers or electrohydraulic or electric valve actuators are provided. Furthermore, it has proven to be advantageous to be able to influence the opening and closing timing of the gas-exchange valves during the operation of the internal-combustion engine. It is also desirable to be able to influence the opening or closing timing of the inlet or outlet valves separately, in order to be able to set, for example, a targeted definite valve overlap. By setting the opening or closing timing of the gas-exchange valves as a function of the current characteristic field of the engine, for example, the current engine speed or the current load, the specific fuel consumption can be reduced, the exhaust behavior can be influenced positively, and the engine efficiency, the maximum torque, and the maximum output can be increased.
The described variability in the gas-exchange timing is implemented by a relative change of the phase position of the camshaft relative to the crankshaft. Here, the camshaft is in drive connection with the crankshaft usually via a chain, belt, gear, or similarly acting drive concept. A camshaft adjuster, which transfers the torque from the crankshaft to the camshaft, is mounted between the chain, belt, or gear drive driven by the crankshaft. Here, this device is embodied such that during the operation of the internal-combustion engine, the phase position is reliably held between the crankshaft and camshaft and, when desired, the camshaft can be rotated into a certain angular range relative to the crankshaft.
In internal-combustion engines with camshafts for the inlet and outlet valves, these camshafts can each be equipped with a camshaft adjuster. Therefore, the opening and closing times of the inlet and outlet gas-exchange valves are shifted in time relative to each other and the overlapping of the timing is set as desired.
The seat of modern camshaft adjusters is generally located on the drive-side end of the camshaft. It comprises a driving wheel fixed to the crankshaft, a driven part fixed to the camshaft, and an adjusting mechanism transferring the torque from the driving wheel to the driven part. The driving wheel can be embodied as a chain, belt, or gear, and is connected in a rotationally fixed manner to the crankshaft by means of a chain, belt or gear drive. The adjusting mechanism can be operated electrically, hydraulically, or pneumatically.
In hydraulically operated camshaft adjusters, one differentiates between so-called axial piston adjusters and rotary piston adjusters.
In the axial piston adjusters, the driving wheel connects to a piston by means of helical gearing. Furthermore, the piston connects to the driven part likewise via helical gearing. The piston separates a hollow chamber formed by the driven part and the driving wheel into two compression chambers arranged axially relative to each other. Now, if one compression chamber is pressurized with a hydraulic medium, for example, motor oil, while the other compression chamber is connected to an oil outlet, then the piston is displaced in the axial direction. This axial displacement creates a relative rotation of the driving wheel relative to the driven part and thus the camshaft relative to the crankshaft by means of the two helical gearing pairs.
In a rotary piston adjuster, the driving wheel is connected in a rotationally fixed manner to a stator. The stator and the driven part are arranged concentrically relative to each other. The radial intermediate space between these two components includes at least one, but usually several, hollow chambers spaced apart in the circumferential direction. The hollow chambers are bounded in a pressure-tight manner by side covers in the axial direction. A vane connected to the driven part extends into each of these hollow chambers. This vane divides each hollow chamber into two compression chambers. Through targeted connection of the individual compression chambers to a hydraulic medium pump or to a hydraulic medium outlet, the phase of the camshaft relative to the crankshaft can be set or held.
To control the camshaft adjuster, sensors detect the characteristic data of the engine, such as, for example, the load state and the engine speed. This data is fed to an electronic controller, which controls the adjusting motor of the camshaft adjuster or the inflow and outflow of hydraulic medium to the various compression chambers after comparison of the data with a characteristic data field of the internal-combustion engine.
A device for changing the timing of an internal-combustion engine is known, for example, from JP 03 026 815 A. This document describes the controlled drive of an engine provided with two banks of cylinders arranged in the shape of a V relative to each other. The engine is provided with an intake camshaft and an exhaust camshaft for each cylinder bank. The intake camshafts are driven by the crankshaft via a traction mechanism drive. A hydraulically operated camshaft adjuster is mounted on the drive-side end of each intake camshaft. Each camshaft adjuster is provided with a driving wheel, around which the traction mechanism is tensioned and a driven part fixed to the camshaft is provided. On the end faces of the intake camshafts facing away from the corresponding drive, another driving wheel for another traction mechanism drive is mounted, by means of which the corresponding exhaust camshaft is driven. To enable an adjustment of the timing between intake camshaft and exhaust camshaft, each exhaust camshaft is provided on the drive side with a camshaft adjuster.
A disadvantageous effect in this embodiment is that for driving the two intake camshafts, two camshaft adjusters are needed, wherein each camshaft adjuster is mounted on one of the intake camshafts. The use of two camshaft adjusters leads to higher costs, greater weight, and increased assembly expense for the controlled drive. Another disadvantage is that for the use of hydraulic camshaft adjusters, two pressurized hydraulic medium supply systems must be provided in the internal-combustion engine. This leads to increased adaptation expense for the surrounding components of the camshaft adjuster, such as, for example, the camshaft or the cylinder head.